Starfish Diversity in the Wenlock of England

Abstract

Although their record extends back to the Early Ordovician, the occurrence of fossil starfish (Echinodermata: Asteroidea) is dependent almost exclusively upon horizons of exceptional preservation. Thus, asteroids found in Silurian obrution deposits of the English Midlands and Welsh Borderlands are particularly significant to an understanding of the early diversity of the group. Six species are described here: Hudsonaster? carectum sp. nov. (Hudsonasteridae), from the lower part of the Lower Elton Formation; and, from the Much Wenlock Limestone Formation, the hudsonasterids Doliaster brachyactis gen. et sp. nov. and Siluraster? ketleyi (Spencer, 1916), the lepidasterids Lepidaster grayi Forbes, 1850 and Lepidactis wenlocki Spencer, 1918, and the palasterinid Palasterina orchilocalia sp. nov. Though few in number, they show a diverse range of body morphologies when compared with Ordovician taxa: L. wenlocki had long, slender rays when fully grown whereas D. brachyactis is the first asteroid with the short-rayed body form of extant cushion stars. Most distinctive of all is L. grayi, the earliest multiradiate taxon known, all complete specimens of which have 13 rays. This morphological variety is interpreted as indicating that by the Early Silurian starfish were exploiting a wide range of feeding habits and ecological niches.

Evolutionary and ecological significance of Lepidaster grayi, the earliest multiradiate starfish

“…The morphological conservatism of Ordovician asteroids was interpreted by Shackleton (2005) as indicating their ecological restriction, but the presence of Lepidaster in a morphologically varied fauna from the Wenlock of England (see Herringshaw et al ., in press) suggests that Silurian starfish were more ecologically diverse. Too few data are available presently to determine whether the asteroids of the Much Wenlock Limestone Formation were an endemic fauna or representative of a global diversification and, given the poor preservation potential of asteroids, it is also possible that more diverse morphologies, including multiradiate taxa, existed in the Ordovician.…”

“…and it is not until the Silurian that multiradiate species are recorded. Lepidaster grayi from the Much Wenlock Limestone Formation (Wenlock: Homerian) of England is the oldest, with eight specimens known (for full systematics see Herringshaw, Thomas & Smith, in press). The holotype and largest specimen (NHM 40215, Natural History Museum, London) has a diameter of 102 mm; it and the three other most complete specimens all appear to have had 13 rays (Table 1).…”

Asterozoans from the Ludlow Series (upper Silurian) of Leintwardine, Herefordshire, UK

“…Herringshaw et al . () retained the species within Arisaigaster . However, the arrangement of the InfMM in relation to the axillaries, when viewed dorsally, on NHMUK E 13153 does not resemble the ventral morphology of A .…”

“…); the Llandoverian of southern Ontario, Canada (Eckert ); the Sheinwoodian Rochester Formation of western New York, USA and southern Ontario, Canada (Schuchert ; Ruedemann ; Taylor & Brett ); the Homerian Much Wenlock Limestone Formation of the West Midlands, England (Spencer 1916, 1918, 1922; Herringshaw et al . ); and the Gorstian to Ludfordian Bannisdale Formation and Kirkby Moor Formation starfish beds of Westmorland, England (Marr ; Spencer 1918, 1922, 1940; Lewis et al . ).…”

“…By contrast, the Wenlockian deposits at Dudley, the most diverse echinoderm‐Lagerstätte of the British Silurian, only exhibits a relatively limited asterozoan component, with just asteroid taxa documented (five species; Herringshaw et al . ; Donovan et al . ).…”